When network packet arrives at a typical network interface card (NIC), the NIC moves the data into pre-allocated network buffers in system main memory. From there the data is read into the CPU cache so that it can be checksummed (assuming of course that the protocol in use requires checksums. Some, like IPX, do not.). Once the data has been fully processed by the protocol stack, it can then be moved into its final destination in memory. Since the CPU is moving the data, and must read the destination cache line in before it can fill it and write it back out, this involves at a minimum 2 more trips across the system memory bus. In short, the best one can hope for is that the data will get moved across the system memory bus 4 times before it arrives in its final destination. It can, and does, get worse. If the data happens to get invalidated from system cache after it has been checksummed, then it must get pulled back across the memory bus before it can be moved to its final destination. Finally, on some systems, including Windows NT 4.0, the data gets copied yet another time while being moved up the protocol stack. In NT 4.0, this occurs between the miniport driver interface and the protocol driver interface. This can add up to a whopping 8 trips across the system memory bus (the 4 trips described above, plus the move to replenish the cache, plus 3 more to copy from the miniport to the protocol driver). That's enough to bring even today's advanced memory busses to their knees.
In all but the original move from the NIC to system memory, the system CPU is responsible for moving the data. This is particularly expensive because while the CPU is moving this data it can do nothing else. While moving the data the CPU is typically stalled waiting for the relatively slow memory to satisfy its read and write requests. A CPU, which can execute an instruction every 5 nanoseconds, must now wait as long as several hundred nanoseconds for the memory controller to respond before it can begin its next instruction. Even today's advanced pipelining technology doesn't help in these situations because that relies on the CPU being able to do useful work while it waits for the memory controller to respond. If the only thing the CPU has to look forward to for the next several hundred instructions is more data moves, then the CPU ultimately gets reduced to the speed of the memory controller.
Moving all this data with the CPU slows the system down even after the data has been moved. Since both the source and destination cache lines must be pulled into the CPU cache when the data is moved, more than 3 k of instructions and or data resident in the CPU cache must be flushed or invalidated for every 1500 byte frame. This is of course assuming a combined instruction and data second level cache, as is the case with the Pentium processors. After the data has been moved, the former resident of the cache will likely need to be pulled back in, stalling the CPU even when we are not performing network processing. Ideally a system would never have to bring network frames into the CPU cache, instead reserving that precious commodity for instructions and data that are referenced repeatedly and frequently.
But the data movement is not the only drain on the CPU. There is also a fair amount of processing that must be done by the protocol stack software. The most obvious expense is calculating the checksum for each TCP segment (or UDP datagram). Beyond this, however, there is other processing to be done as well. The TCP connection object must be located when a given TCP segment arrives, IP header checksums must be calculated, there are buffer and memory management issues, and finally there is also the significant expense of interrupt processing, discussed below.
A 64 k server message block (SMB) request (write or read-reply) is typically made up of 44 TCP segments when running over Ethernet, which has a 1500 byte maximum transmission unit (MTU). Each of these segments may result in an interrupt to the CPU. Furthermore, since TCP must acknowledge (ACK) all of this incoming data, it's possible to get another 44 transmit-complete interrupts as a result of sending out the TCP acknowledgements. While this is possible, it is not terribly likely. Delayed ACK timers allow us to acknowledge more than one segment at a time. And delays in interrupt processing may mean that we are able to process more than one incoming network frame per interrupt. Nevertheless, even if we assume 4 incoming frames per input, and an acknowledgement for every 2 segments (as is typical per the ACK-every-other-segment property of TCP), we are still left with 33 interrupts per 64 k SMB request.
Interrupts tend to be very costly to the system. Often when a system is interrupted, important information must be flushed or invalidated from the system cache so that the interrupt routine instructions, and needed data can be pulled into the cache. Since the CPU will return to its prior location after the interrupt, it is likely that the information flushed from the cache will immediately need to be pulled back into the cache. What's more, interrupts force a pipeline flush in today's advanced processors. While the processor pipeline is an extremely efficient way of improving CPU performance, it can be expensive to get going after it has been flushed. Finally, each of these interrupts results in expensive register accesses across the peripheral bus (PCI).
We noted earlier that when the CPU has to access system memory, it may be stalled for several hundred nanoseconds. When it has to read from PCI, it may be stalled for many microseconds. This happens every time the CPU takes an interrupt from a standard NIC. The first thing the CPU must do when it receives one of these interrupts is to read the NIC Interrupt Status Register (ISR) from PCI to determine the cause of the interrupt. The most troubling thing about this is that since interrupt lines are shared on PC-based systems, we may have to perform this expensive PCI read even when the interrupt is not meant for us.
Other peripheral bus inefficiencies also exist. Typical NICs operate using descriptor rings. When a frame arrives, the NIC reads a receive descriptor from system memory to determine where to place the data. Once the data has been moved to main memory, the descriptor is then written back out to system memory with status about the received frame. Transmit operates in a similar fashion. The CPU must notify that NIC that it has a new transmit. The NIC will read the descriptor to locate the data, read the data itself, and then write the descriptor back with status about the send. Typically on transmits the NIC will then read the next expected descriptor to see if any more data needs to be sent. In short, each receive or transmit frame results in 3 or 4 separate PCI reads or writes, not counting the status register read.